1. Field of the Invention
The present invention relates to a method and apparatus for altering the temporal and spatial shape of an optical pulse. Pulse stretchers based on volume holographic chirped reflection gratings (VHCRG) are used for increasing the temporal length of an optical pulse prior to amplification by an optical amplifier. After amplification, the optical pulse is temporally recompressed by a pulse compressor in order to achieve a short duration pulse. During the process of stretching and compressing, the spatial shape of the pulse can be distorted by the volume grating. It is desirable to obtain a mean to produce a beam spatial profile that is clean, i.e. free of spatial distortion after the stretching and compression steps by diffraction from a chirped reflecting volume holographic grating.
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2. Background Art
FIG. 1 illustrates a state-of-the-art pulse stretcher/compressor pair that produces a high power short pulse. A seed oscillator optical pulse 100 is collimated and directed to a pulse stretcher comprised of two dispersive diffraction gratings 110 and a pair of lenses positioned in between. The diffraction gratings 110 are placed one focal length away from the lenses. The stretched pulse 120 is amplified by an optical amplifier 130, whose output produces a high power stretched pulse 140. The high power long pulse is shortened by a compressor that uses two dispersive diffraction gratings 150. The output of the compressor is a short and intense pulse 160.
The compressor/stretcher based on dispersive grating are bulky due to the small angular dispersion that can be achieved. In contrast, a pulse stretcher/compressor based on non-dispersive volume holographic chirped reflection gratings (VHCRG) is several times smaller. FIG. 2 illustrates the concept. A seed oscillator optical pulse 200 is collimated and directed to a pulse stretcher that is comprised of a VHCRG. The input aperture is typically several square millimeters. The VHCRG can be made out of different thick holographic materials such as photo-thermal glass (PTR) or crystals which have a high peak power damage threshold. Commercial PTR VHCRG typically have several hundreds of MW/cm2 damage threshold for 20 ns pulses at 20 Hz repetition rate near 1 μm. FIG. 3 illustrates a damage threshold measurement for commercial PTR volume holographic material.
In PTR holographic glass, a small DC index change arises between the top and bottom of the VHCRG. Absorption of the recording beam during the recording process creates an uneven exposure in the direction of the recording beam throughout the thickness of the material. In holographic photo-thermo refractive glass for example, this exposure change creates a small DC index change of the order of 10−4.
The DC index change is related to the illumination exposure and thus along the thickness of the sample, the DC index change varies continuously. The DC index gradient affects the propagation of a collimated beam. FIG. 4 illustrates this effect. An undistorted collimated beam 400 with a beam size of the order of the thickness of the VHCRG 410 will be diffracted into beam 420 in the direction of the DC index gradient thus deforming the spatial profile of the incident beam. The output beam profile 430 is shown in FIG. 4. The extent of the angular deflection can be approximated by the following formula: α≈(∂n/∂z) L/n, where a is the deflection angle, (∂n/∂z) the index of refraction gradient, L the length of the VHCRG and n its average index of refraction. For example, the expected deflection angle in the case of an index gradient of 10−4/mm, length L of 30 mm and average index of 1.5 yields a deflection angle of 2 mrad. Because the diffracted beam propagates twice the length L of the VHCRG (by reflection), the total deflection angle becomes 4 mrad. After a free space propagation of only 25 cm, a 1 mm diameter pulse diffracted by the VHCRG will be elongated in one direction (the direction of the DC index gradient) by 1 mm. The extent of the oblong spatial beam profile of the diffracted beam 420 matches the above quantitative explanation. Although small, the effect on the spatial beam profile is detrimental for proper amplification of the stretched pulse. It is also detrimental when the recompressed pulse needs to be close to distortion free for applications such as but not limited to thin film photovoltaic scribing, precise machining and ablation.
In order to increase the time delay, while maintaining the same length VHCRG, a double pass configuration with a VHCRG is used. FIG. 5 illustrates the method. A seed oscillator optical pulse 500 is collimated and directed to a pulse stretcher that is comprised of a VHCRG 510 and a flat mirror 520. The angular positioning of the mirror is such that the diffracted beam is reflected and counter propagating. The double pass in the VHCRG 510 increases the time delay by a factor 2 with respect to the single pass configuration illustrated in FIG. 2. However, the beam distortion is amplified by a factor 2 as well. FIG. 6 illustrates this effect. The incident beam is diffracted by the VHCRG 600 and reflected by a flat mirror 610 to produce a counter-propagating beam which is in turn re-diffracted by the VHCRG 600 to produce beam 620. At each diffraction, the deflection increase towards the DC index gradient.